Weather by month
Koh Samui weather, month by month
Twelve months of temperature, rain and sea data from 1991-2020 climate normals, so you can pick your dates with your eyes open.
Samui's wet season runs October through December, not the June-to-August southwest monsoon that soaks Phuket and the Andaman coast. February and March are the driest, sunniest months and the safest bet for a trip. April and May run hottest. The table below breaks each month down by temperature, rainfall, sunshine and sea warmth.
A year on one axis
Five rows share the same twelve columns: rain, rain days, sun, temperature as a low-to-high span, and sea warmth. Line them up and the wet, dry and hot stretches show themselves.
Month
- J Jan
- F Feb
- M Mar
- A Apr
- M May
- J Jun
- J Jul
- A Aug
- S Sep
- O Oct
- N Nov
- D Dec
Four windows the data draws
Each bracket is computed straight from the climate normals above, not picked by feel. The rule behind each one is printed underneath it.
-
Driest February – April
The longest run of months whose rainfall sits below the year's median of 122.5mm.
-
Wettest October – January
The longest run of months whose rainfall sits above the year's median of 122.5mm.
-
Sunniest February – March
The longest run of months within half an hour of the sunniest month's 7.5 sun hours a day.
-
Warmest sea April – August
The longest run of months within half a degree of the year's warmest sea reading, 30°C.
Every month, one row each
- January 125 mm Rain from the Nov-Dec monsoon is tapering off, but still 8 rain days; sea holds at 28°C.
- February 65 mm Driest and sunniest month: 65mm over just 4 days, 7.5 sun hours, the best window to book.
- March 115 mm Only 5 rain days but 115mm falls hard when it does; sun holds at 7.5 hours, sea warms to 29°C.
- April 85 mm Hot and mostly dry, 85mm over 6 days; sea reaches 30°C and stays there into May.
- May 130 mm Highest average high of the year at 32.9°C, and rain returns too: 130mm over 11 days.
- June 135 mm Southwest monsoon rain sets in, 135mm over 10 days, though Samui stays drier than the Andaman side.
- July 115 mm 11 rain days and 115mm, sun down to 5 hours; the Andaman coast fares far worse this month.
- August 100 mm Driest of the mid-year months, 100mm over 10 days, with sea still warm at 29.5°C.
- September 120 mm Rain edges up to 120mm over 11 days as the wetter October-November stretch approaches.
- October 295 mm Rain jumps to 295mm over 16 days and sun drops to 4.5 hours, the least sun of any month.
- November 445 mm Wettest month by far, 445mm over 16 days; skip it if rain is a dealbreaker.
- December 265 mm Rain eases to 265mm over 12 days as the monsoon winds down; sea starts to cool to 28.5°C.
Koh Samui climate normals by month
| Month | High °C | Low °C | Rain mm | Rain days | Sun h/day | Sea °C | Humidity % | Crowds | Hotel prices |
|---|---|---|---|---|---|---|---|---|---|
| January | 29 | 24.2 | 125 | 8 | 6.5 | 28 | 83 | high | high |
| February | 29.4 | 24.9 | 65 | 4 | 7.5 | 28.5 | 81 | high | high |
| March | 30.4 | 25.4 | 115 | 5 | 7.5 | 29 | 81 | medium | medium |
| April | 31.8 | 26 | 85 | 6 | 6.5 | 30 | 81 | medium | medium |
| May | 32.9 | 25.7 | 130 | 11 | 6 | 30 | 80 | low | low |
| June | 32.7 | 25.3 | 135 | 10 | 5 | 30 | 78 | low | low |
| July | 32.3 | 25.1 | 115 | 11 | 5 | 29.5 | 78 | medium | low |
| August | 32.4 | 25.1 | 100 | 10 | 5 | 29.5 | 78 | medium | medium |
| September | 31.9 | 24.8 | 120 | 11 | 5 | 29 | 80 | low | low |
| October | 30.6 | 24.3 | 295 | 16 | 4.5 | 29 | 85 | low | low |
| November | 29.7 | 24.1 | 445 | 16 | 6 | 29 | 86 | low | low |
| December | 29.3 | 24 | 265 | 12 | 5.5 | 28.5 | 83 | high | high |
What this month means, island-wide
Weather is only the raw number. Here's how it plays out on the coasts, at the waterfalls and on the trails.
-
Sea state
1 of 4
coasts calm
The four coasts don't turn choppy together. North and west stay calm longest; east turns first as the northeast monsoon builds through the back half of the year.
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
A twelve-cell strip showing how many of Samui's four coasts are calm each month January 3 / 4 February 4 / 4 March 4 / 4 April 4 / 4 May 2 / 4 June 2 / 4 July 2 / 4 August 2 / 4 September 1 / 4 October 0 / 4 November 0 / 4 December 2 / 4 -
Waterfall flow
10 of 11
waterfalls running
The calmest, driest months for the beach are the worst for the waterfalls: March has just 2 of 11 running and April has none. October and November flip that, all eleven running at once, the wettest stretch of the year.
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
A twelve-cell strip showing how many of Samui's eleven waterfalls are running strong or good each month January 9 / 11 February 7 / 11 March 2 / 11 April 0 / 11 May 2 / 11 June 4 / 11 July 5 / 11 August 7 / 11 September 10 / 11 October 11 / 11 November 11 / 11 December 9 / 11 -
Dry hiking days
19 of 30
dry days
Rain days cluster in the same stretch the table already shows: fewest in the dry season, most from October through December, so a trail plan should follow the rain-day column above, not the calendar.
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
A twelve-cell strip showing how many days each month see no rain January 23 / 31 February 24 / 28 March 26 / 31 April 24 / 30 May 20 / 31 June 20 / 30 July 20 / 31 August 21 / 31 September 19 / 30 October 15 / 31 November 14 / 30 December 19 / 31
September's reading against the beach, waterfall and hiking data.
Weather questions, answered
Common questions about Samui weather
When is the wet season on Koh Samui?
October through December, not June to August. Samui sits on the Gulf of Thailand side, so the southwest monsoon that hits Phuket and the Andaman coast mid-year mostly misses it; the island gets wet from the northeast monsoon instead.
Does rain mean the whole day is washed out?
No. Even in the wettest months the table above counts rain days, not rain hours; most days still have long dry spells, and showers tend to pass through rather than settle in for the day.
Which month should I avoid?
November, by the numbers: it's the wettest month of the year and sun hours drop with it. October runs a close second.
Is the sea swimmable year round?
The sea-state data above shows the north and west coasts calm most months, with the east and south turning choppy or rough in the wetter stretch. Check the coast-by-coast breakdown for the month you're picking, not just the island average.
How current are these figures?
They're 1991-2020 climate normals, averaged over three decades of daily records, the same standard meteorological offices use. They describe a typical year, not a forecast for any single trip.
Which months are driest and sunniest?
February through April for dry weather, February and March for sun, per the computed windows above. Both are built straight from the rainfall and sunshine normals, not picked by feel.
Plan your dates around the weather
See how these months line up with flights, hotel prices and crowds in our guide to when to visit Koh Samui.
When to visit